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Dental caries is a chronic invasive disease of the mineralised tissues of the teeth. The organisms reported to be responsible include Streptococcus mutans, S. sobrinus, Lactobacillus casei and Actinomyces species. Current methods of treatment involve mechanical removal of large amounts of healthy, as well as diseased, tissue. Killing of the causative bacteria in situ would reduce the need to remove sound tissue resulting in a better prognosis for affected teeth. A possible approach to achieving this may be the use of lethal photosensitisation. This involves treating the target bacteria with photosensitising compounds and exposing them to laser light resulting in the generation of cytotoxic species such as singlet oxygen and free radicals. The purpose of this study was to investigate the lethal photosensitisation of cariogenic bacteria in vitro. Toluidine blue O (TBO) and aluminium disulphonated phthalocyanine (AIPcS2) were used in conjunction with a helium neon (HeNe) laser and a gallium aluminium arsenide (GaAIAs) laser respectively. Parameters such as photosensitiser concentration, pre-irradiation time, light dose, pH and the physiological state of the bacteria were studied. In order to determine the mechanism of kill, attempts were made to identify the photochemically-induced mediators of cell damage and their targets. Lethal photosensitisation of bacteria in biofilms and in plaque samples was also carried out and the effect of dentine and collagen was also determined to mimic more closely the conditions in vivo. Large numbers of the target bacteria were killed at energy doses of less than 0.6 J of HeNe laser light and 0.9 J of GaAIAs laser light in the presence of the appropriate photosensitiser and this was unaffected by pre-irradiation time. Exposure of the sensitised target bacteria to both HeNe and GaAIAs laser light for varying exposure times resulted in an energy dose related decrease in their viability. L.casei appeared to be slightly more resistant at pH 4.5, and in it's lag phase, requiring twice the energy dose to kill appreciable numbers. Membrane damage occurred in photosensitised S. mutans cells and the mediators of this reaction appeared to be reactive oxygen species. All four bacteria could be killed in biofilms when sensitised by TBO or AIPcS2 following exposure to 0.2 J or 0.9 J of HeNe or GaAIAs laser light respectively. Following irradiation of sensitised supragingival plaque samples substantial reductions in the total anaerobic count, as well as the viable streptococci and actinomyces, were observed. Substantial kills (107 cfu) were also achieved when dentine slices were interposed between S. mutans and the laser light and when the sensitised bacteria were embedded in a collagen matrix. The results of this in vitro study imply that lethal photosensitisation, if effective in vivo, may be useful in the treatment of caries.